(1) Field of the Invention
The present invention relates to a plastic multilayer vessel having an excellent oxygen permeation resistance under conditions where water and heat act. More particularly, the present invention relates to a plastic multilayer vessel having an excellent oxygen permeation resistance at the heat sterilization and with the lapse of time after the heat sterilization.
(2) Description of the Related Art
Metal cans, glass bottles and various plastic vessels have been used as packaging vessels. Plastic vessels are used in various fields because the weight is light, the impact resistance is high and the manufacturing cost is small.
In case of metal cans or glass bottles, permeation of oxygen through the vessel is zero. In contrast, in case of plastic vessels, permeation of oxygen through the vessel wall cannot be neglected, and a problem arises with respect to the storage of a content of the vessel.
As means for overcoming this disadvantage, there has been proposed and used a plastic vessel having a multilayer structure comprising at least one layer composed of a resin having an oxygen permeation resistance, such as an ethylene/vinyl alcohol copolymer.
From old, an oxygen scavenger has been used for removing oxygen from the interior of a vessel, and Japanese Examined Patent Publication No. 62-1824 proposes application of this oxygen scavenger to the vessel wall. Namely, according to this conventional technique, a layer of an oxygen-permeable resin in which an oxygen scavenger composed mainly of a reducing substance is incorporated is laminated with a layer having an oxygen-barrier property, wherey a packaging laminate structure is formed.
According to this conventional technique, the oxygen scavenger present in the vessel wall absorbs oxygen in the vessel to maintain a highly oxygen-free state in the vessel. However, the packaging vessel of this type is still insufficient in that the oxygen concentration cannot be controlled to a low level under conditions in which water and heat simultaneously act, namely under heat sterilization conditions.
An oxygen-barrier resin such as an ethylene/vinyl alcohol copolymer has, in general, a moisture-absorbing property, and the resin also has such a property that the oxygen permeation coefficient increases when it absorbs moisture. Accordingly, a multilayer structure comprising an intermediate layer of an oxygen barrier resin and inner and outer layers of a moisture-resistant resin such as an olefin resin disposed on both the sides of the intermediate layer is generally adopted. However, under the above-mentioned conditions in which water and heat simultaneously act, permeation of water through the olefin resin layer is caused, and it is considered that the amount of oxygen in the vessel increases by increase of the oxygen gas permeability by the absorption of moisture and also by increase of the oxygen permeation by the elevation of the temperature.
As the means for overcoming the above-mentioned defect, we proposed a plastic multilayer vessel composed of a laminate comprising an intermediate layer formed of a resin composition comprising a gas-barrier thermoplastic resin having a low oxygen permeability and a high water absorption and an oxygen scavenger incorporated therein and layers of a moisture-resistant thermoplastic resin disposed on both the sides of the intermediate layer (see Japanese Patent Application No. 63-105446).
This plastic multilayer vessel is epoch-making in that the permeation of oxygen during the retort sterilization and the permeation of oxygen with the lapse of time after the retort sterilization can be controlled, but this plastic multilayer vessel is still insufficient in that the oxygen-absorbing property is relatively low when the partial pressure of oxygen is low, and the oxygen-absorbing speed is relatively low.